1. Technical Field
The present invention relates to a device that displays an image by modulating a light from a light source via plural light modulation elements, and more particularly, to an image display device and a projector suitable to achieve a wider luminance dynamic range and a greater number of gradation steps.
2. Related Art
Improvements of an image quality in recent years with electronic display devices, such as an LCD (Liquid Crystal Display), an EL (Electro-luminescence) display, a plasma display, a CRT (Cathode Ray Tube), and a projector, are remarkable. A device with the ability comparable to the characteristics of human vision as to resolution and color gamut is now being achieved. However, the reproducible range of the luminance dynamic range is limited to a range of the order of 1 to 102 [nit], and the gradation steps is normally expressed by 8 bits. On the other hand, the luminance dynamic range perceivable by the human vision at a time is in a range of the order of 10−2 to 10−4 [nit], and luminance discretion is 0.2 [nit], which is equivalent to 12 bits in terms of gradation steps. When a display image of today's display device is viewed through such vision characteristics, the narrowness of the luminance dynamic range is outstanding. In addition, because gradation in a shadow portion and a highlight portion is insufficient, the viewer is not completely satisfied with a display image in terms of reality and a visual impact.
Meanwhile, the mainstream of the CG (Computer Graphics) used in movies, games, etc. is to pursue the reality of a picture by providing display data with a luminance dynamic range and the gradation characteristic closer to those of the human vision (hereinafter, such display data is referred to as HDR (High Dynamic Range) display data). However, because the ability of a display device to display the HDR display data is inadequate, there is a problem that the expression capability inherent to CG contents is not fully exerted.
Further, for the next-generation OS (Operating System), the adaptation of a 16-bit color space is planned, and the dynamic range and the gradation steps will be widened and increased markedly in comparison with the current 8-bit color space. Hence, the need for an electronic display device capable of fully expressing the 16-bit color space by achieving a high dynamic range and a greater number of gradation steps is expected to increase.
Of the display devices, a projection type display device (projector), such as a liquid crystal projector and a DLP™ (Digital Light Processing) projector, is able to display an image on a large screen, and is therefore a display device effective when rendering a display image with reality and a visual impact. In this field, proposals as follows have been made to solve the problems discussed above.
The technique disclosed, for example, in JP-T-2004-523001 is an example of a display device achieving a high dynamic range. This display device includes a light source, a second light modulation element that modulates luminance of a light in all the wavelength regions, and a first light modulation element that modulates luminance of a light in each of the wavelength regions for three primary colors, RGB, among wavelength regions of the light. A light from the light source is modulated by the second light modulation element to form a desired luminance distribution, and the resultant optical image is formed on the display surface of the first light modulation element to modulate colors, so that a secondary-modulated light is projected. All the pixels in the second light modulation element and the first light modulation element are controlled individually according to a first control value and a second control value determined on the basis of the HDR display data. A transmissive modulation element having a pixel structure or a segment structure in which transmissivity can be controlled independently and capable of controlling a two-dimensional transmissivity distribution is used as the light modulation element. A good example is a liquid crystal light valve. Alternatively, the transmissive modulation element may be replaced with a reflective modulation element. A good example in this case is a DMD (Digital Micromirror Device).
In the case where a light modulation element whose transmissivity is 0.2% for a dark display and 60% for a bright display is used, the luminance dynamic range of a single unit of the light modulation element is 60/0.2=300. The display device is equivalent to an array of light modulation elements having the luminance dynamic range of 300 and aligned optically in series. The display device is therefore able to achieve a luminance dynamic range of 300×300=90000. The same applies to the gradation steps, and gradation steps exceeding 8 bits can be achieved by aligning light modulation elements having 8-bit gradation optically in series.
JP-A-2001-100689 is also an example of the related art.
Incidentally, the shape of an opening in the pixel of the liquid crystal light valve is not rotational symmetric about the central axis of the image display surface. Hence, in the related art, when the light source, the first light modulation element, the image forming system, and the second light modulation element are disposed on the optical axis, the shape of an opening in the unit pixel for an optical image of the first light modulation element formed by the image forming system does not match with the shape of an opening in the unit pixel of the second light modulation element. Part of a light exiting from the first light modulation element is therefore shielded at the opening of the second light modulation element. This raises a problem that efficiency for utilization of light is reduced.